THE EXCEPTION TO “THE RULE”
Phyllodesmium acanthorhinum is a rarely sighted, though widely distributed, species first recorded from Okinawa by Bob Bolland in 1987. The 15 mm long specimen pictured here was found in the Mooloolah River, southern Queensland, Australia in 3 m of water.
It had long been thought that species of Phyllodesmium only have smooth rhinophores, or at the very most somewhat wrinkled. That was a fair enough “rule” too considering that every described member of the genus up until mid 2014 supported that conclusion.
Enter Phyllodesmium acanthorhinum. This species breaks that axiom. Described in a 2014 paper by Moore & Gosliner, Phyllodesmium acanthorhinum does not have smooth rhinophores with the authors stating: “…there are numerous, yellow-cream–colored tubercles on the entire surface of the rhinophores that lead to a yellow-cream, pointed tip.” The paper also contains a table of comparative features and of the 23 species of Phyllodesmium compared therein only Phyllodesmium acanthorhinum has ornamented rhinophores. The authors have acknowledged this unusual revelation through their choice of specific epithet that describes “…the spiny appearance of the papillae on the surface of the rhinophores.”
The ornamented rhinophores of Phyllodesmium acanthorhinum.
There are 27 described species of Phyllodesmium and at least a further 14 awaiting the attention of taxonomists.
So in your identification searches certainly follow “the rules” but keep an open mind for the exceptions as “the rules” are only invented as a guide based on interpretation of current knowledge. In taxonomy there will always be exceptions. It is in the nature of evolution.
Reference: Additions to the Genus Phyllodesmium, with a Phylogenetic Analysis and its Implications to the Evolution of Symbiosis, Elizabeth Moore & Terrence Gosliner, The Veliger 51(4):237–251, 2014
David A. Mullins – October 2019